feat(caddy): add Caddyfile TextMate grammar and enable Shiki syntax highlighting

Adds lib/grammars/caddyfile.json with scopes for directives, named-matchers
(@prefix), placeholders, strings, and comments. Updates /caddy page to use
createHighlighter with the local grammar instead of the nginx fallback.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/grammars/caddyfile.json

@@ -0,0 +1,83 @@
+{
+  "name": "caddyfile",
+  "scopeName": "source.Caddyfile",
+  "fileTypes": ["Caddyfile"],
+  "patterns": [
+    { "include": "#comment" },
+    { "include": "#site-address" },
+    { "include": "#named-matcher-def" },
+    { "include": "#named-matcher-ref" },
+    { "include": "#directive" },
+    { "include": "#placeholder" },
+    { "include": "#string-double" },
+    { "include": "#string-backtick" },
+    { "include": "#number" },
+    { "include": "#braces" }
+  ],
+  "repository": {
+    "comment": {
+      "name": "comment.line.number-sign.caddyfile",
+      "match": "#.*$"
+    },
+    "site-address": {
+      "name": "entity.name.section.caddyfile",
+      "match": "^(?:https?://)?[a-zA-Z0-9][a-zA-Z0-9.*-]*(?::[0-9]+)?(?=\\s*(?:\\{|,|$))"
+    },
+    "named-matcher-def": {
+      "name": "entity.other.attribute-name.caddyfile",
+      "match": "@[a-zA-Z_][a-zA-Z0-9_-]*(?=\\s)"
+    },
+    "named-matcher-ref": {
+      "name": "entity.other.attribute-name.caddyfile",
+      "match": "(?<=\\s)@[a-zA-Z_][a-zA-Z0-9_-]*"
+    },
+    "directive": {
+      "patterns": [
+        {
+          "name": "keyword.control.caddyfile",
+          "match": "\\b(reverse_proxy|encode|file_server|handle_errors|handle_path|handle|root|header|request_header|response_header|redir|respond|rewrite|uri|try_files|php_fastcgi|push|templates|basicauth|forward_auth|map|vars|log|tls|bind|import|snippet|abort|error|static_response|acme_server|invoke)\\b"
+        },
+        {
+          "name": "support.function.caddyfile",
+          "match": "\\b(on_demand|off|auto|internal|force|strip_prefix|replace|path_regexp|method|host|header_regexp|remote_ip|client_ip|not|query|cookie|expression|path|protocol|vars_regexp|file|jwt|geo_ip)\\b"
+        },
+        {
+          "name": "keyword.other.option.caddyfile",
+          "match": "\\b(auto_https|admin|debug|grace_period|shutdown_delay|servers|storage|order|email|acme_ca|acme_ca_root|acme_eab|ocsp_stapling|key_type|cert_issuer|local_certs|skip_install_trust|renew_interval|check_interval|persistent_key|insecure_secrets_log|prefer_wildcard|resolvers|max_size|retention|format|output|level|sampling|include|exclude|dial|upstream|transport|lb_policy|health_uri|health_interval|health_timeout|health_status|health_body|flush_interval|buffer_requests|buffer_responses|max_buffer_size|trusted_proxies|to|from|prefix|replacements|gzip|zstd|br)\\b"
+        }
+      ]
+    },
+    "placeholder": {
+      "name": "variable.other.caddyfile",
+      "match": "\\{(?:http\\.(?:request|response|vars|regexp|handlers)|tls|env|vars|system|time|rand|counter|uuid|path|query|header|cookie|form|file|dir|args|blocks|labels|err|http)[^}]*\\}"
+    },
+    "string-double": {
+      "name": "string.quoted.double.caddyfile",
+      "begin": "\"",
+      "end": "\"",
+      "patterns": [
+        {
+          "name": "constant.character.escape.caddyfile",
+          "match": "\\\\."
+        },
+        {
+          "name": "variable.other.caddyfile",
+          "match": "\\{[^}]+\\}"
+        }
+      ]
+    },
+    "string-backtick": {
+      "name": "string.quoted.other.caddyfile",
+      "begin": "`",
+      "end": "`"
+    },
+    "number": {
+      "name": "constant.numeric.caddyfile",
+      "match": "\\b[0-9]+(?:\\.[0-9]+)?(?:s|ms|m|h|d|kb|mb|gb)?\\b"
+    },
+    "braces": {
+      "name": "punctuation.section.block.caddyfile",
+      "match": "[{}]"
+    }
+  }
+}